1. Field of the Invention
This invention relates to a metal halide discharge lamp containing sodium to getter halogen. More particularly, this invention relates to a high intensity metal halide discharge lamp containing at least one ionizable metal halide, such as an iodide, and sodium metal to getter the excess halogen.
2. Background of the Disclosure
High intensity metal halide arc discharge lamps are well known to those skilled in the art, dating back to 1966 when Reiling added halides of various light-emitting metals to a high pressure mercury lamp to improve the color and efficacy of the lamp as is disclosed in U.S. Pat. No. 3,234,421. Since then metal halide lamps have become commercially useful for general illumination. Light-emitting metals favored by Reiling were sodium, thallium and indium in the form of iodides. This combination had the advantage of giving a lamp starting voltage almost as low as that of a mercury vapor lamp, thus permitting interchangeability of metal halide with mercury lamps in the same sockets. A later U.S. Pat. No. 3,407,327 to Koury et al issued in 1968, proposed as additive metals sodium, scandium and thorium which produces light of better quality, but requires a higher starting voltage so that the lamp is not generally interchangeable with mercury vapor lamps. Combinations of halogens such as sodium and scandium iodides with or without thallium iodide are still widely used and preferred for general illumination metal halide lamps. Unfortunately, sodium and scandium iodides are hygroscopic which results in introducing moisture into the lamp arc tube or arc chamber during the manufacturing process. This results in the formation of mercury iodide which causes hard starting requiring higher starting and operating voltages and also poorer lumen maintenance. In one manufacturing process, the lamps are dosed with mercury as liquid and with the iodides of Na, Sc and Th in pellet form. In this process, it is practically unavoidable that some hydrolysis reaction occurs due to absorption of moisture from the atmosphere by the pellets in transferring them to the lamp envelope. The metal halide dose comprising NaI, ScI.sub.3 and ThI.sub.4 is extremely hygroscopic and even very low levels of moisture will result in some hydrolysis. The hydrolysis results in conversion of metal halide to oxide with release of HI, for example: EQU 2ScI.sub.3 +3H.sub.2 O.fwdarw.Sc.sub.2 O.sub.3 +6HI
The HI reacts with mercury to form HgI.sub.2 which is relatively unstable at high temperatures, and when the lamp warms up, the HgI.sub.2 decomposes and releases free iodine. This all occurs in a short period of time, usually within the first few hours of lamp operation. Some excess iodine or other halogen is also frequently found in the dosing materials, possibly as a by-product of the synthesis of these materials. The result is a lamp which frequently contains excess iodine from the start.
To overcome this problem of free iodine formation, prior art lamps generally contain a metal to getter the excess iodine and/or other halogen, along with other impurities such as water, oxygen and nitrogen. Such metals have included cadmium, scandium, thallium, zinc and thorium. However, scandium and thorium are expensive and difficult to control as to the proper amount, because they don't readily form an amalgam with mercury and must therefore be introduced into the arc chamber as pieces of metal. Thorium is also radioactive. Zinc, cadmium and thallium are undesirable because they result in the formation of volatile halides which produce higher halogen partial pressures in the arc than would be present if scandium or thorium had been used as the getter. The higher halogen partial pressure can result in more rapid tungsten transport from the electrodes to the arc chamber wall with concomitant wall blackening and lumen loss. Thus, there is still a need for a more effective getter in such lamps.